Electrochromic devices are devices that change light (and heat) transmission properties in response to voltage applied across the device. Electrochromic devices can be fabricated which electrically switch between transparent and translucent states (where the transmitted light is colored). Furthermore, certain transition metal hydride electrochromic devices can be fabricated which switch between transparent and reflective states. Electrochromic devices are incorporated in a range of products, including architectural windows, rear-view mirrors, and protective glass for museum display cases.
A prior art electrochromic device 100 is represented in FIGS. 1 & 2, which show a schematic representation of the electrochromic device illustrating ion conduction between anode and cathode, and a cross-sectional representation of the electrochromic device, respectively. See Granqvist, C.-G., Nature Materials, v5, n2, February 2006, p 89-90; C.-G. Granqvist Handbook of Inorganic Electrochromic Materials, Elsevier, 1995; and U.S. Pat. No. 5,995,271 to Zieba et al. The device 100 comprises a glass substrate 110, lower transparent conductive oxide (TCO) layer 120, a cathode 130, a solid electrolyte 140, a counter electrode (anode) 150, upper TCO layer 160, a protective coating 170, a first electrical contact 180 (to the lower TCO layer 120), and a second electrical contact 190 (to the upper TCO layer 160). Furthermore, there may be a diffusion barrier layer (not shown) between the glass substrate 110 and the lower TCO layer 120, to reduce the diffusion of ions from the glass substrate into the TCO layer, and vice versa. Note that the component layers are not drawn to scale in the electrochromic devices shown in FIGS. 1 & 2. For example, a typical glass substrate is of the order of a millimeter thick and a typical electrochromic device covers the fully exposed area of the architectural glass, or rear-view mirror, for example. Other substrate materials may be used, for example plastics such as polyimide (PI), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). Typical component layer thicknesses are given in the table below:
Component LayerThickness (microns)lower TCO layer0.1 to 1.0cathode0.03 to 1.0solid electrolyte0.005 to 5counter electrode0.03 to 1.0upper TCO layer0.1 to 1.0diffusion barrier layer0.1 to 1.0
Switching from a transparent to a colored state, for example, occurs when ions (such as lithium or hydrogen ions) are driven from the counter electrode 150, through the (non electrically conductive) solid electrolyte 140, to the cathode 130. The counter electrode 150 is an ion storage film, and the cathode 130 is electrochromic—providing the desired change in light transmission properties. It is also possible for the counter electrode 150 to function as the electrochromic layer if this layer undergoes an “anodic coloration,” where the layer changes from transparent to colored with de-intercalation of the ion. In this case, the cathode becomes the counter electrode. One can also create greater contrast by combining the effects of both electrodes. A more detailed discussion of the functioning of electrochromic devices is found in Granqvist, C.-G., Nature Materials, v5, n2, February 2006, p 89-90 and C.-G. Granqvist Handbook of Inorganic Electrochromic Materials, Elsevier, 1995. For the device to function properly, the lower TCO layer 120 and the cathode 130 must be electrically isolated from the counter electrode 150 and upper TCO layer 160. Electrical contact to external driver circuits is made through the first and second electrical contacts 180 and 190.
Lithium intercalation and transport are determining factors in the performance of an electrochromic (EC) device. Currently, the most commonly used cathode and anode materials are WO3 and variations of NiO films, respectively, in which lithium intercalation and transport in the anode are the bottlenecks. Thus, the electrochemical reaction speed and device performance are limited. There is a need for anode materials which promote the anodic reaction.
Furthermore, there is a need for improved anode materials in other electrochemical devices, such as thin film batteries (TFB).